In recent years, attention is given to optical communication systems that send and receive data by using illumination light of an LED (light emitting diode) illumination device. The optical communication systems using illumination light can be constructed by utilizing already existing infrastructures.
The optical communication system which uses visible light such as illumination light as a communication medium has advantages such as being able to clearly define a range where communication is possible and being able to communicate in an area where use of radio waves is restricted. Accordingly, new uses have been studied, other than a use for supplementing communication by existing wireless communication systems utilizing radio waves and existing infrared communication applications utilizing infrared rays. As a use of the optical communication system utilizing visible light, a use utilizing LED light sources used in outdoor facilities such as a signal lamp and a digital signage is conceivable. For example, new uses such as high-speed data communication, data communication for positioning to assist and control the driving of a vehicle, and data communication for acquiring the information on flow lines of customers in a commercial facility are conceivable.
There is a representative example of the communication method utilizing illumination light that assigns data 1 or 0 (i.e., 1/0) to ON or OFF of a pulse signal, converts ON or OFF of the pulse signal to blinking of an LED, and thereby sends (light-sends) an optical signal (modulated light). In this method, the optical signal reception device detects the optical signal by using a light receiving element and restores the pulse signal from a blinking pattern of the optical signal.
FIG. 1 is a diagram showing a configuration example of an optical communication system utilizing illumination light. In the optical communication system shown in FIG. 1, the optical signal sent from an illumination device #1 is received by a plurality of receiving terminals α, β, and γ. The optical communication system shown in FIG. 1 is applied to a case that common data is distributed to the plurality of receiving terminals α, β, and γ in a communication area, and other cases.
FIG. 2 is a diagram showing another configuration example of the optical communication system utilizing illumination light. In the optical communication system shown in FIG. 2, the optical signal sent from an illumination device #1 is received by a receiving terminal α; the optical signal sent from an illumination device #2 is received by a receiving terminal β; and the optical signal sent from an illumination device #3 is received by a receiving terminal γ. The communication system shown in FIG. 2 is applied to a case that different sets of data are distributed from the plurality of illumination devices #1, #2, and #3 to the plurality of receiving terminals α, β, and γ, respectively, and other cases.
Generally, illumination devices placed to ensure illumination are not supposed to be used as communication devices. In the optical communication system shown in FIG. 1, since data distributed to the receiving terminals α, β, and γ are common data, the interference of sent optical signals does not occur. In the optical communication system shown in FIG. 2, however, there is a possibility that the receiving terminals α, β, and γ detect not only desired optical signals sent from their respective closest illumination devices #1, #2, and #3, but also an unwanted optical signal sent from another nearby illumination device. In other words, in the optical communication system shown in FIG. 2, since the receiving terminals α, β, and γ detect even an unwanted optical signal, there is a possibility that desired optical signals cannot be accurately detected.
As a countermeasure, patent reference 1 proposes an optical communication system in which a transmission device generates a signal string obtained by performing an inverse discrete Fourier transform of each set of a predetermined number of information bits and sends the signal string as an optical signal according to the light emission intensities of a plurality of light emitting elements, and a reception device receives the optical signal sent from the transmission device with a plurality of light receiving elements and performs a discrete Fourier transform of the received signal to restore data. The optical communication system of patent reference 1 eliminates interference caused by unwanted optical signals sent from transmission devices other than a desired transmission device in that way.